1. Field of the Invention
This invention relates generally to the field of Swimmer Entrapment Avoidance, and more specifically to the means for precluding swimmer entrapment, entanglement or evisceration due to suction drains in swimming pools, spas, and the like; with a hydraulically independent sensor that anticipates a user's danger.
2. Description of Prior Art
The Consumer Product Safety Commission (CPSC) has reported over many years that there are dozens of deaths and grave injuries each year in the US, mostly young children, due to the suction entrapment hazards of swimming pools, wading pools and spas. The CPSC has recently set up testing facilities for Safety Vacuum Release Systems (SVRS); products now on the market intended to rapidly reduce suction and release an entrapped person.
All SVRS devices now sense an increase in suction, near the pump inlet, that occurs when a person blocks all or a major part of a remote suction drain. None can anticipate the event, and that is a serious flaw in swimmer protection.
Thus, the prior art is only capable of catch and release; not really avoidance, as specified in ANSI/APSP-7 2006, Standard for Suction Entrapment Avoidance in Swimming Pools, Wading Pools, Spas, Hot Tubs, and Catch Basins.
The potential and actual hazards due to underwater suction drains include evisceration that can occur in a fraction of a second, if the drain cover is missing; hair entanglement, and limb, body, or mechanical entrapment, all as defined in ANSI/APSP-7 2006.
In addition to the tragic results mentioned there are large societal costs related to long term medical treatment of the injured, major awards and expenses of litigation, inhibiting business activity, and reducing opportunity for the public to enjoy the fitness, health and recreation benefits of safe water facilities whether public or private.
The main problem with conventional entrapment avoidance sensors is that they are constrained to allow a very significant increase in the suction, due to actual entrapment, before taking corrective action. This allows a potential victim to approach the drain closely without a significant increase in the suction being sensed. Only when the suction port is mostly blocked by the victims body or limb does a large increase in suction suddenly occur. Under these conditions a small child may be partially or totally eviscerated in an extremely short period of time. Some tests reported in the literature indicate that damage can be done within a small fraction of a second, when the short distance to complete the drain sealing is covered and a very high degree of vacuum is thereby allowed to occur momentarily. Furthermore, hair entanglement occurs without a major increase in suction at all.
When a deep pool drain cover is damaged or missing, a lethal hazard for limb or body entrapment is created. A missing drain cover is also an invitation to limb entrapment because instant swelling of arm tissues under the pipe vacuum condition may not allow extrication even if an SVRS does function as expected.
In a shallow pool, as at children's wading pools, a damaged or missing drain cover creates a lethal hazard for drowning or evisceration. No SVRS can sense that condition and take protective action prior to an entrapment.
Hair entanglement in a drain cover happens very quickly; and is also not likely to trigger an SVRS. Fatalities have occurred in this manner.
Some other prior art deficiencies may be summarized as follows:
                Present SVRS also have a major weakness in terms of field reliability over years of time with no requirements for periodic, automatic calibration, testing, and traceability of such tests.Experience with outdoor installations shows that there are three primary hazards to safety and control system reliable operation:        Lightning and induced power surge damage occurs rapidly and can easily go undetected without frequent testing.        Corrosion is slow but steady, and reliability is unpredictable without frequent testing.        Lack of self calibration and self test capability.        
Furthermore, all SVRS devices are hydraulically dependent sensors, so that changing flow circulation conditions due to poor filter maintenance, pump speed changes, changes in valve settings, cleaning system variables, dual drains with one blocked, etc. can have a serious effect upon the suction sensor functioning properly when it must. Additionally, fail-safe principles in design, fabrication and installation are not applied in any systematic, verifiable, way in these SVRS devices.